Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/strings/ea35743.kt

@@ -0,0 +1,6 @@
+val Int.test = "test"
+
+fun box(): String {
+    val x = "a ${1.test}"
+    return if (x == "a test") "OK" else "Fail $x"
+}